PROJECT SUMMARY Inhibitory receptors, such as PD1 and LAG3, synergize to exert critical cell intrinsic control of T cell function, but also contribute to the development, homeostasis and function of regulatory T cells (Tregs). Inhibitory receptors are highly expressed on exhausted tumor-infiltrating lymphocytes (TILs), rendering PD1 and LAG3 barriers to effective anti-tumor immunity. We have shown that combinatorial PD1/LAG3 targeted immunotherapy induces almost complete remission in mice with pre-existing tumors. AIM 1: What is the relative and synergistic contributions of PD1 and LAG3 in preventing effective anti- tumor immunity by CD8+ T cells? We hypothesize that ?PD1 and LAG3 synergize to have a dominant effect on CD8+ TILs, thereby mediating tumor-induced tolerance?. We will use CD8+-restricted Cre-mediated PD1/LAG3 deletion to assess their relative and mechanistic contribution to tumor-induced CD8+ T cell exhaustion, using transplantable and genetically-induced models of human cancer, especially melanoma. AIM 2: What is the contribution of PD1 and LAG3 in modulating intratumoral Treg function? We hypothesize that ?PD1 and LAG3 limit Treg function but enforce stability in the tumor microenvironment?. We will determine if PD1/LAG3 contribute to Treg function in the tumor microenvironment using mice with restricted PD1/LAG3 deletion in Tregs. PPG Interactions: Project 2 will collaborate with Project 1 to determine if inhibitory receptor loss in tumor- bearing mice induces an autoimmune insult, and Project 3 to compare T cell exhaustion induced by tumors and chronic viral infections. In all Aims, Project 2 will utilize Core B to obtain mice, Core A to obtain statistical support, Core C for transcriptional analysis, and Core D for immunohistological analysis.